Gas separation modules are commonly used to selectively separate a particular gas from a gas mixture. Two of the most common gas separation modules are polymer membranes and metallic composites. Polymer membranes can provide an effective and cost-efficient option for separating a gas at low temperatures. Where separations must be performed in conjunction with high-temperature processing, however, polymer membranes are generally unsuitable because they tend to thermally decompose.
The development of high-temperature processing, along with tighter environmental regulations, requires utilization of gas separation modules that provide high flux, high selectivity of separation, and the ability to operate at elevated temperatures. Instead of polymers, metallic composite modules can be employed to serve these needs. A composite gas separation module can consist of a metallic membrane having selective gas permeability mounted on a porous substrate. Alternatively, a gas separation module can be formed purely of a hydrogen-selective metal such as palladium. However, such a module can be very expensive to produce and can lack the mechanical strength that can be required for high pressure and/or high temperature applications.
An area of high-temperature gas separation that is of particular interest is the separation and purification of hydrogen gas from a reaction gas mixture. A composite module for selectively separating hydrogen gas at high temperatures can include a palladium (Pd) membrane. Ideally, the palladium membrane is permeable to hydrogen but not to other gases. When hydrogen gas (H2) contacts the membrane, the hydrogen molecules dissociate and hydrogen atoms diffuse into the membrane. Accordingly, hydrogen can selectively pass from a surrounding atmosphere through the palladium membrane. The selectively separated hydrogen atoms then reassociate into H2 gas and pass into a volume on the opposite side of the module.
Typical hydrogen-selective metal membranes used in composite gas separation modules must be free of defects and/or pinholes that breach the metal layer to prevent the migration of undesired gases through the metal membrane. However, thick gas-selective metal membranes, e.g., palladium membranes, generally are very expensive. In addition, the use of thick membranes to separate gas mixtures usually results in low fluxes of gas(es).
Therefore, a need exists for composite gas separation modules (and methods for their fabrication) that overcome or minimize the above-referenced problems.